Shock-Strength Determination With Seeded and Seedless Laser Methods

Two noninvasive laser diagnostics were independently used to measure time-averaged and spatially resolved pressure change across a two-dimensional (2D) shock wave. The first method is Doppler global velocimetry (DGV) which uses water seeding and generates 2D maps of three-orthogonal components of velocity. A DGV-measured change in flow direction behind an oblique shock provides an indirect determination of pressure change across the shock, when used with the known incoming Mach number and ideal shock relations (or Prandtl–Meyer equations for an expansion fan). This approach was demonstrated at Mach 2 on 2D shock and expansion waves generated from a flat plate. This technique also works for temperature change (as well as pressure) and for normal shocks (as well as oblique). The second method, laser-induced thermal acoustics (LITA), is a seedless approach that was used to generate 1D spatial profiles of streamwise Mach number, sound speed, pressure and temperature over the same oblique waves. Excellent agreement was obtained between DGV and LITA, suggesting that either technique is viable for shock-strength measurement.